Yes... but it will likely require more time in the monster arena... and that you haven't saved (via the record book) since before you promoted...

Go to the record book, and click on swap details. This swaps the details that you have right now, with the details that you had when you last saved.

If you saved at level 10, still a swordsman, then just click on the Dark Fruit in your inventory. But if you neglected the save option, then you're going to have to work your way back up to level 10 again...

If you saved after the promotion, however... you're out of luck...

Follow your dreams, you never know where they'll lead you.

Being defeated is only a temporary condition. Giving up is what makes it permanant.